The present invention relates to hydrostatic transmissions and control systems therefor, and more particularly, to such control systems which make it possible for the vehicle operator to select among multiple modes of operation.
Although the present invention may be used with any type of variable displacement pump or motor in which the displacement of the unit is controlled by a fluid pressure actuated servomechanism, the invention is especially suited for use with axial piston swashplate pumps and motors, and will be described in connection therewith.
It is well known in variable displacement axial piston swashplate pumps and motors to control the position of the swashplate by porting fluid under pressure to one of two servos having pistons attached to the swashplate (the stroking servo) and by porting fluid in the other servo (the destroking servo) to drain. The control of such fluid is governed generally by a control valve having a control spool which has a feedback linkage connected to the swashplate, or to the servo-pistons. Such control valves are typically referred to as "manual controllers", because the position of the swashplate ultimately corresponds to the position of a manual control handle.
Such manual controllers are widely used, especially in the mobile hydraulics market, in connection with hydrostatic transmissions which provide vehicle propulsion. Such manual controllers are simple, inexpensive, and reliable. However, such manual controllers provide only a single operating mode or "gain" rate (i.e., the change in swashplate position for a given change in control handle position).
Many vehicles which are propelled by hydrostatic transmissions need to be able to operate in more than one operating mode, or at more than one gain rate. For example, vehicles used on construction sites need to be able to operate over the full range of available pump displacements and vehicle speeds when moving from one site to another (i.e., the "transport mode"). However, while such a vehicle is being used at a work site, it needs to operate over only a very narrow range of vehicle speeds and pump displacements (i.e., an "inching" mode). Control of swashplate position and vehicle speed over a very narrow range, using a standard manual controller, is typically too difficult to be acceptable to the operators of such vehicles.
In an attempt to overcome the problems described above, the prior art has developed various inching valves and inching controls, whereby it would be possible for the vehicle operator to use all, or most of, the normal control handle movement to select only a narrow range of swashplate displacements. The most common arrangement in the prior art is that set forth in U.S. Pat. No. 3,715,017, wherein there is provided an inching valve which reduces the control pressure communicated from the charge pump to the manual controller. Theoretically, reducing the charge pressure should, for any given control handle position, reduce the swashplate angle, which is the objective of an inching control system. However, inching control systems which operate on the principle of reducing charge pressure have not been especially successful commercially, and do not actually provide true inching control of the type desired.
In a properly designed servo system for an axial piston pump, one of the criteria for selecting the charge pressure to be used (typically, 250 or 300 psi), is to have a pressure high enough that it is able to provide a predictable swashplate versus control handle relationship, independent of system pressure, which exerts an opposing moment on the swashplate. In an inching control system, it would be desirable to have a gain rate about 1/3 that in the normal operating mode. If the control pressure is reduced substantially, in an attempt to achieve such a reduced gain rate, the swashplate angle versus control handle position will now be a function of system pressure. To illustrate the problem with such inching control systems more specifically, at relatively high system pressure (e.g., 4,000 psi), movement of the control handle over its full range may result in displacement of the swashplate over only a small portion of its full range of displacement, which is the desired result of an inching control system. However, at relatively low system pressure (e.g., 1,000 psi), movement of the control handle over its full range may still result in movement of the swashplate over its full range of displacements, the same as in a normal operating mode.